Abstract: The overall goal of this proposal is to optimize the selection and the identification of drugs that can ablate leukemia stem cells (LSCs) that are not effectively ablated during induction therapy. Acute myeloid leukemia (AML) treatment is to date unsatisfactory where most patients will die from their disease despite achieving initial complete remission (CR). Even under very aggressive multi-agent chemotherapy regimens and myeloablative allogeneic stem cell transplantation, relapse rates are high. The last 30 years have seen only marginal improvements in durable remission rates. Clearly, new therapeutic approaches are needed. Increasing evidence suggests that AML is originated and maintained by a population of cells known as LSCs, which are also resistant to standard chemotherapy regimens and are thereby available to provide a reservoir of cells that drive disease relapse. Indeed, studies have shown that a high percentage of phenotypically-defined LSCs correlates to especially poor prognosis. Therefore, we propose here that new therapies should center around new therapeutic endpoints such as the ablation of chemoresistant populations of LSCs. To achieve this goal, we hypothesize that LSCs are frequently present during remission at that they should be targeted during consolidation therapy. The aims of this proposal are to advance the targeting of LSCs and reduction of relapse by: (i) determining whether ex vivo treatments of LSCs more realistically reflect therapeutic outcome in patients compared to AML blast populations;(ii) defining the gene expression signatures of drug sensitivity and drug resistance of LSCs to identify better therapies;and (iii) identifying cell lines that best mimic the chemosensitivity of LSCs to have a readily available LSC surrogate in drug screens. Public Health Relevance: Acute myeloid leukemia (AML) is a fatal disease for most patients and novel treatment strategies are urgently needed. Most new agents are tested in patients with relapsed or refractory disease, but these patients generally have highly resistant disease that either will not respond to any treatment or proliferates so quickly that the drugs have no opportunity to work. This proposal describes a novel strategy to expand drug development and clinical trials in AML to specifically target the resistant residual leukemia cells of patients who seem to be in remission but are, in reality, destined to relapse.